Segment Information (Details) - Information About Reportable Segments - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating (loss) income	$ (6,329,000)	$ 305,141,000	$ 287,997,000
Operating Segments
Segment Reporting Information [Line Items]
Revenues	3,311,494,000	2,444,540,000	2,163,056,000
Total segment gross contribution	2,000,386,000	1,512,000,000	1,334,547,000
Operating Segments | Research
Segment Reporting Information [Line Items]
Revenues	2,471,280,000	1,857,001,000	1,614,904,000
Total segment gross contribution	1,653,014,000	1,285,611,000	1,117,534,000
Operating Segments | Consulting
Segment Reporting Information [Line Items]
Revenues	327,661,000	318,934,000	296,317,000
Total segment gross contribution	93,643,000	89,734,000	86,486,000
Operating Segments | Events
Segment Reporting Information [Line Items]
Revenues	337,903,000	268,605,000	251,835,000
Total segment gross contribution	163,480,000	136,655,000	130,527,000
Operating Segments | Talent Assessment & Other
Segment Reporting Information [Line Items]
Revenues	174,650,000	0	0
Total segment gross contribution	90,249,000	0	0
Corporate and Reconciling Items
Segment Reporting Information [Line Items]
Corporate and other expenses	$ (2,006,715,000)	$ (1,206,859,000)	$ (1,046,550,000)